Data synchronization is very important for the applications in power system, especially for current differential protection. Some external synchronization mechanism such as echo method or GPS is used widely by the protection system to ensure the data synchronization in practical applications. However, the reliability of echo method depends on the symmetry of communication channels and it is not always symmetrical in practical. And the signal of GPS is not always reliable enough. It will influence the reliability of current differential protection markedly and even may cause mal-operation in some conditions.
The main problem of existing echo method is that its operation is severely affected by the channel symmetry status. Along with the development and improvement of modern power communication network, self-healing ring network or optical fiber channel with variable routing is gradually increasing. It causes new problems to optical fiber current differential protection for transmission lines. Existing data synchronization method assumes that the time latency of both sending and receiving channels are the same. Thus once the asymmetrical channel switching, the delays of sending and receiving channels are no longer equal, and thus the reliability of protection devices will be affected due to the data out of synchronization.
The main problem of synchronization by GPS is that GPS signal and device is not reliable enough to meet the requirement of protection system in practical, although GPS can provide accurate, absolute and synchronized time for all substations theoretically.
To overcome the problems of the existing external synchronization methods (echo method and GPS method), some patents such as CN101651324A, JP8037724A, JP2008125251 and JP2007068325, have disclosed methods based on load current instead of external synchronization mechanism. Although these patents have different specific embodiments, their basic principle can be described as following.
See FIG. 1, the methods in above mentioned patents are based on the fact that the measured load current vectors from the two terminals should be the same (same amplitude and reverse phase) in normal operation status. If the current phase difference measured from the two terminals are not reverse (180°), it means the data synchronization is wrong, and the phase difference from 180° can be used to synchronize the measurements. This principle can also be implemented by comparing the wave shift of the two load current waves.
This method is based on the measured currents instead of echo message or GPS signals. Thus, it will not be affect by channel asymmetry or reliability of GPS in principle. But phase angle of load currents in light load or no load conditions cannot be measured accurately. Thus, the load current based synchronization method cannot deal with light load or no load conditions.
And another shortcoming of this method is that it can only deal with two-terminal transmission line and cannot synchronize the measurements for the transmission line with more than two terminals.
As shown in FIG. 2, for two-terminal system, since the phase angles of the two load current vectors should be the reverse, there is only one choice to decide the phase difference between the two currents. Thus, accurate synchronization for two-terminal system can be finished. But for three-terminal system, it has two choices for the relative phase difference among the three current vectors. And for four-terminal or more terminal systems, there are even countless choices for the phase angle. Thus, the unique relative angle difference among these vectors cannot be determined. That means the measurements for the transmission line with more than two terminals by the said load current based synchronization method cannot be synchronized.
And in addition, these patens based on load currents have not considered the problem of more than half cycle synchronization error. Because the load current is periodical wave, if the synchronization error may be bigger than half cycle, the synchronization method cannot decide which direction is the right one for correcting the error.
In summation, for all the methods in above mentioned patents, although existing load current based synchronization method will not be influenced by asymmetrical channels, they cannot deal with light load or no load conditions, cannot deal with transmission lines with more than two terminals, and cannot deal with more than half cycle synchronization error.
Our previous patent application PCT/CN2009/076347 disclosed a method and apparatus for detecting communication channel delay asymmetry between transmission line protection devices. The method comprises: calculating, repeatedly, clock disparity between clocks of the protection devices and communication delays at different paths of the communication channel; comparing the latest calculated clock disparity and communication delays with previously calculated clock disparities and communication delays, respectively; determining a channel switching has happened if a change of the calculated clock disparity exceeds a first threshold, or a change of the calculated communication delays for any path exceeds a second threshold; and determining the channel delays as asymmetrical if a difference between the calculated communication delays of the different paths after the channel switching exceeds a third threshold. The improved echo method is based on the characteristic of echo method and local clocks instead of measurements. Thus, it can deal with the problems of asymmetrical channel switching including the cases of >½ cycle synchronization errors. However, the improved echo method may have the problem of accumulative error if there is lots of channel switching.